Method for treating a spinal deformity

ABSTRACT

A dynamic stabilization device is disclosed. The device includes a dual spring member comprising an outer spring and an inner spring that have approximately equal working lengths. The dynamic stabilization device is also configured so that the dual spring member does not undergo stresses greater than an effective fatigue limit that is related to a fatigue limit of the spring. Methods for treating a deformity of a spine using a dynamic stabilization device are also disclosed.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/840,731, filed on Aug. 17, 2007, now U.S. Pat. No. 8,080,038, issuedDec. 20, 2011, which is herein incorporated by reference in itsentirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to spinal implantations and inparticular to a dynamic stabilization device configured for the spine.

2. Description of Related Art

Methods of spinal stabilization have previously been proposed. Previousmethods have incorporated various components configured to provide sometype of flexibility. Jahng et al. (U.S. patent number 2005/0203513)teaches a spinal stabilization device. The stabilization device includesa longitudinal member having first and second ends as well as a flexiblesection disposed between the first and second ends. Jahng furtherteaches a cross sectional profile for the flexible section that isdifferent than the cross sectional profiles of the first and secondends. Jahng teaches a flexible section that includes spiral cut groovesto improve flexibility.

Generally, spiral cut grooves may not provide the same degree offlexibility and support as a spring. Methods of spinal stabilizationincluding screws have also been proposed. Timm et al. (U.S. patentnumber 2005/0171543) is directed to a system for effecting multi-levelspine stabilization. Timm teaches a system including a plurality ofpedicle screws that are joined by rods. Timm further teaches that atleast one of the rods includes a dynamic stabilizing member. Timmteaches an inner first spring and an outer second spring. In the Timmdesign, the inner first spring is generally disposed within the outersecond spring. Timm teaches springs that are not connected directly tothe stabilization device at their ends, but instead are free springsdisposed between two surfaces.

Colleran et al. (U.S. patent number 2006/0036240), teaches a system andmethod for dynamic skeletal stabilization. Colleran teaches two screwsthat are each associated with a separate bracing portion. Colleranteaches a spring and two stops that allow the two bracing portions tomove longitudinally with respect to one another. This provides somedegree of movement between the two screws. In the Colleran design, oneof the brace portions may also bend.

Rothman et al. (U.S. patent number 2006/0229612) teaches a method forvertebral stabilization using sleeved springs. Rothman teaches a springthat is disposed between two anchoring elements. Rothman further teachessleeve elements that cover the ends of the springs. The sleeve elementsinclude an inner surface configured to receive the springs and an outersurface configured to engage the anchoring elements. The sleeve elementsinclude ends that serve as stops for the spring.

These methods and systems incorporating springs as dynamic componentshave several drawbacks. First, the methods and systems taught here lackwell defined connection points for the springs, and instead rely onstops or sleeve assisted stops. Also, in these systems, the springs maynot facilitate inward tension as the springs are stretched, norfacilitate outward tension that is associated with spring compression.

Methods of attaching rods to bone screws have been previously proposed.Tornier et al. (U.S. Pat. No. 5,662,651) teaches an external or internalfixator for repairing fractures of arthroplasties of the skeleton.Tornier teaches an implant screw that is connected to a support that isangularly indexed with respect to the screw. The support includes acavity configured to receive a connecting rod. Tornier further teaches alocking screw that is fastened into place into the support member,thereby locking the connecting rod into place. The Tornier design hasseveral drawbacks. The connecting rod is attached to a support that isseparate from the screw, providing a potentially weakened connectionbetween the connecting rod and the screw. Additionally, the lockingscrew does not include provisions to easily receive the surface of theconnecting rod. Furthermore, Tornier does not teach a drive receivingsurface used to install the screw.

There is a need in the art for a design that solves many of the problemsof the prior art.

SUMMARY OF THE INVENTION

A dynamic stabilization device configured for the spine is disclosed. Inone aspect, the invention provides a dynamic stabilization deviceconfigured for implantation into a spine, comprising: an outer springand an inner spring, wherein the inner spring is disposed within theouter spring; and where the outer spring has a first working length thatis equal to a second working length of the inner spring.

In another aspect, the inner spring is configured to attach to a firstthreaded portion of a first rod of the dynamic stabilization device.

In another aspect, the outer spring is configured to attach to a secondthreaded portion of a second rod of the stabilization device.

In another aspect, the first rod is attached to a first anchorconfigured for implantation into a first vertebra.

In another aspect, the second rod is attached to a second anchorconfigured for implantation into a second vertebra.

In another aspect, the invention provides a dynamic stabilization deviceconfigured for implantation into a spine, comprising: a dual springmember including a first end; a first rod including a first threadedportion; and where the first end of the dual spring member is configuredto mechanically attach to the first threaded portion of the first rod.

In another aspect, a second end of the dual spring member is configuredto attach to a second threaded portion associated with a second rod.

In another aspect, the dual spring member comprises an outer spring andan inner spring.

In another aspect, the outer spring is configured to connect to a firstouter threaded portion of the first rod.

In another aspect, the inner spring is configured to connect to a firstinner threaded portion of the first rod.

In another aspect, the inner spring and the outer spring are connectedat the second end of the dual spring member.

In another aspect, the inner spring and the outer spring haveapproximately equal working lengths.

In another aspect, the inner spring and the outer spring are configuredto experience stresses that are less than a fatigue limit associatedwith the inner spring and the outer spring.

In another aspect, the invention provides a dynamic stabilization deviceconfigured for implantation into a spine, comprising: a dual springmember including an outer spring and an inner spring, the dual springmember having a fatigue limit; and where the dynamic stabilizationdevice is configured so that stresses applied to the dual spring memberare always below an effective fatigue limit.

In another aspect, the fatigue limit is associated with a stressselected from a group consisting essentially of shear stresses, tensionsstresses, compression stresses, torsional stresses, rotational stressesand any combination thereof.

In another aspect, the effective fatigue limit is between 10 percent ofthe fatigue limit and 75 percent of the fatigue limit.

In another aspect, the dual spring member has a life expectancy that iseffectively indefinite.

In another aspect, the dual spring member is connected to a first outerthreaded portion at a first end.

In another aspect, the dual spring member comprises an outer spring andan inner spring.

In another aspect, the outer spring has a first working length that isapproximately equal to a second working length associated with the innerspring.

In another aspect, the inner spring comprises an inner coil and whereinthe outer spring comprises an outer coil, and wherein the inner coil isdifferent than the outer coil.

In another aspect, the inner spring has a first shape and wherein theouter spring has a second shape, and wherein the first shape isdifferent than the second shape.

In another aspect, the invention provides a dynamic stabilization deviceconfigured for implantation into a spine, comprising: an anchorconfigured for implantation into a bone, including a proximal portionand a distal portion; the proximal portion including threadingconfigured to penetrate a vertebra; the distal portion including a drivereceiving surface; the distal portion including a first set of recessesconfigured to receive ridges associated with a rod; and where the firstset of recesses is disposed within a drive receiving surface and whereina cap configured to cover the distal portion includes a second set ofrecesses configured to receive the ridges.

In another aspect, the rod is associated with a dual spring member.

In another aspect, the dual spring member includes an inner spring andan outer spring;

In another aspect, the inner spring and the outer spring haveapproximately equal working lengths.

In another aspect, the dual spring member is configured to attach to athreaded portion of the rod.

In another aspect, the maximum stress applied to the dual spring memberis below an effective fatigue limit that is less than half of a fatiguelimit.

In another aspect, the inner spring and the outer spring arecontinuously formed at a first end of the dual spring member and at asecond end of the dual spring member.

In another aspect, the inner spring and the outer spring are physicallyseparated at a first end of the dual spring member and at a second endof the dual spring member.

In another aspect, the inner spring and the outer spring arecontinuously formed at a first end of the dual spring member and whereinthe inner spring and the outer spring are physically separated at asecond end of the dual spring member.

In another aspect, a surface of the rod is textured.

In another aspect, the ridges have a shape selected from the groupconsisting essentially of sinusoidal ridges, box-like ridges, triangularridges, rounded ridges, and any combination thereof.

In another aspect, the dynamic stabilization device may be used todynamically treat or correct various deformities.

In another aspect, the anchor is a screw.

In another aspect, the anchor is a hook.

In another aspect, the rod includes threading.

In another aspect, the rod includes a locking member configured toengage at least one spring.

In another aspect, the invention provides a dynamic stabilization systemconfigured for implantation into a spine, comprising: a first dynamicstabilization device configured to be attached to adjacent vertebrae,the first dynamic stabilization device being pre-stressed and includinga first residual stress; a second dynamic stabilization deviceconfigured to be attached to the adjacent vertebrae, the second dynamicstabilization device being pre-stressed and including a second residualstress; and wherein the first and second dynamic stabilization devicesare configured to apply a force to the adjacent vertebrae therebyadjusting the relative positions of the adjacent vertebrae.

In another aspect, wherein the first residual stress is different thanthe second residual stress.

In another aspect, wherein the first residual stress is tension and thesecond residual stress is compression, and wherein a rotational force isapplied to the adjacent vertebrae whereby scoliosis may be treated.

In another aspect, wherein the first residual stress is substantiallyequal to the second residual stress, and wherein a translational forceis applied to the adjacent vertebrae whereby spondylolisthesis may betreated.

In another aspect, wherein the first residual stress is substantiallyequal to the second residual stress, and wherein a flexion force isapplied to the adjacent vertebrae whereby spinal stenosis may betreated.

Other systems, methods, features and advantages of the invention willbe, or will become apparent to one with skill in the art uponexamination of the following figures and detailed description. It isintended that all such additional systems, methods, features andadvantages be included within this description, be within the scope ofthe invention, and be protected by the following claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention can be better understood with reference to the followingdrawings and description. The components in the figures are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the invention. Moreover, in the figures, likereference numerals designate corresponding parts throughout thedifferent views.

FIG. 1 is a side view of a preferred embodiment of a spine with adynamic stabilization device;

FIG. 2 is an exploded isometric view of a preferred embodiment of adynamic stabilization device;

FIG. 3 is an isometric view of a preferred embodiment of a dynamicstabilization device;

FIG. 4 is an exploded isometric view of an alternative embodiment of adynamic stabilization device;

FIG. 5 is an isometric view of an alternative embodiment of a dynamicstabilization device;

FIG. 6 is an isometric view of a preferred embodiment of a dual springmember;

FIG. 7 is an isometric view of an alternative embodiment of a dualspring member;

FIG. 8 is an isometric view of an alternative embodiment of a dualspring member;

FIG. 9 is a preferred embodiment of a dual spring member attaching totwo rods;

FIG. 10 is a preferred embodiment of a dual spring member attached totwo rods;

FIG. 11 is an alternative embodiment of a dual spring member attachingto two rods;

FIG. 12 is an alternative embodiment of a dual spring member attached totwo rods;

FIG. 13 is a cross sectional view of an alternative embodiment of a dualspring member attached to two rods;

FIG. 14 is a side view of a preferred embodiment of a spine with adynamic stabilization device including a dual spring member experiencingbending;

FIG. 15 is a side view of a preferred embodiment of a spine with adynamic stabilization device including a dual spring member experiencingbending;

FIG. 16 is an exemplary embodiment of a relationship between fatiguestrength and number of stress cycles for a dual spring member;

FIG. 17 is a preferred embodiment of a connection between a rod and ananchor;

FIG. 18 is an alternative embodiment of a connection between a rod andan anchor;

FIG. 19 is an alternative embodiment of a connection between a rod andan anchor;

FIG. 20 is a preferred embodiment of two dynamic stabilization devicesundergoing pre-tensioning and pre-compression;

FIG. 21 is a preferred embodiment of two dynamic stabilization devicesconfigured to correct scoliosis; and

FIG. 22 is a preferred embodiment of two dynamic stabilization deviceswith dual spring members in unstressed positions.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a side view of a preferred embodiment of dynamic stabilizationdevice 100. In this embodiment, dynamic stabilization device 100 isconfigured to attach to spine 101. Generally, dynamic stabilizationdevice 100 may be applied in surgeries intended to address discdegenerative diseases, spinal stenosis, spondylolisthesis, scoliosis, aswell as other spinal problems. Dynamic stabilization device 100 mayallow for dynamic support to spine 101 to provide immediatepostoperative stability when used in the case of disc replacement ornucleus replacement.

In this embodiment, dynamic stabilization device 100 is configured toattach to first vertebra 102 and second vertebra 104. First vertebra 102and second vertebra 104 are further associated with spinal disc 106.Spinal disc 106 is disposed between vertebrae 102 and 104. In someembodiments, spinal disc 106 could be surgically altered, includingreduction in size. In other embodiments, spinal disc 106 may be a discimplant or disc replacement configured to provide support betweenvertebrae 102 and 104 following the removal of a spinal disc.

FIGS. 2 and 3 are intended to illustrate the various componentsassociated with dynamic stabilization device 100 in a preferredembodiment. Preferably, dynamic stabilization device 100 includes firstanchor 202 and second anchor 204. In some embodiments, anchors 202 and204 are bone screws. In other embodiments, anchors 202 and 204 may bepedicle screws that may be configured to implant or screw into pediclesof vertebrae.

First anchor 202 and second anchor 204 preferably include threading 206disposed on distal portion 207 of anchors 202 and 204. Preferably,threading 206 may have a major diameter that is sufficiently largecompared to the minor diameter of threading 206. Furthermore, the pitchof threading 206 is preferably large enough to provide adequate holdingstrength. Using this preferred threading arrangement may provideincreased strength of the connection between anchors 202 and 204 andvertebrae 102 and 104, respectively.

First anchor 202 preferably includes first drive receiving surface 210disposed at proximal portion 209. Preferably, first drive receivingsurface 210 is disposed circumferentially around first anchor 202. Inother embodiments, the drive receiving surface is disposed within anchorhead 214. In the preferred embodiment, first drive receiving surface 210may be a hexagonal surface configured to receive a wrench or ratchet ofsome kind. First drive receiving surface 210 may include first flat side212. Preferably, one or more additional flat sides (not shown) areadjacent to first flat side 212 on first drive receiving surface 210.Using this arrangement, a wrench, socket or other tool may be applied tofirst flat side 212 and one or more additional flat sides (not shown) tomanipulate first anchor 202. In particular, with this arrangement, firstanchor 202 may be drilled or otherwise screwed into place with respectto first vertebra 102.

In other embodiments, first drive receiving surface 210 may have adifferent shape. Furthermore, in some embodiments, additional provisionsmay be provided for driving first anchor 202 into place. In someembodiments, first anchor 202 may include a drive such as are found inthe heads of various types of screws for receiving a screwdriverincluding, but not limited to: slotted drives, Phillips drives, Torxdrives, Hex drives and Robertson drives.

First anchor 202 may also include first anchor head 214. Preferably,first anchor 202 is attached to first drive receiving surface 210. Firstanchor head 214 may include first slotted portion 216. Additionally,first anchor head 214 may include first anchor threading 218.

Preferably, first anchor 202 may be associated with first rod 220. Insome embodiments, first anchor portion 202 may be configured to receivefirst rod 220 at first slotted portion 216 of first anchor head 214.Preferably, first slotted portion 216 is wide enough and deep enough toreceive first rod 220. In an alternative embodiment, first slottedportion 216 and first rod 220 may be joined using an interference fit.In this case, first slotted portion 216 may be just large enough so thatfirst rod 220 can be wedged into first slotted portion 216.

Furthermore, first anchor 202 may be associated with first cap 222.First cap 222 preferably includes first inner cap 224 and first outercap 226. Preferably, first inner cap 224 is configured to fit withinfirst outer cap 226. In some embodiments, first inner cap 224 may beformed integrally with first outer cap 226 resulting in a one piece ormonolithic single cap. Furthermore, first inner cap 224 is preferablyconfigured to receive first rod 220, while first outer cap 226 isconfigured to engage first anchor head 214. An assembled view of firstcap 222, first rod 220 and first anchor 202 can be seen in FIG. 3.Details of this assembly will be explained later in this detaileddescription.

Preferably, second anchor 204 is substantially similar to first anchor202. In particular, second anchor 204 may include second drive receivingsurface 240, including second flat side 242. Furthermore, second anchor204 may include second anchor head 244. Second anchor head 244 may beassociated with second slotted portion 246 and second anchor threading248.

Preferably, second anchor 204 may be associated with second rod 250. Insome embodiments, second anchor 204 may be configured to receive secondrod 250 at second slotted portion 246 of second anchor head 244.Preferably, second slotted portion 246 is wide enough and deep enough toreceive second rod 250. In an alternative embodiment, second slottedportion 256 and second rod 250 may be joined using an interference fit.In this case, second slotted portion 256 may be just large enough sothat second rod 250 can be wedged into second slotted portion 256.

Furthermore, second anchor 204 may be associated with second cap 252.Second cap 252 preferably includes second inner cap 254 and second outercap 256. Preferably, second inner cap 254 is configured to fit withinsecond outer cap 256. In some embodiments, second inner cap 254 may beformed integrally with second outer cap 256 resulting in a one piece ormonolithic single cap. Furthermore, second inner cap 254 is preferablyconfigured to receive second rod 250, while second outer cap 256 isconfigured to engage second anchor head 244. An assembled view of secondcap 252, second rod 250 and second anchor 204 can be seen in FIG. 3.Details of this assembly will be explained later in this detaileddescription.

Preferably, dynamic stabilization device 100 is further associated withdual spring member 270. In some embodiments, dual spring member 270comprises outer spring 272 and inner spring 274. Dual spring member 270preferably includes first end 276 and second end 278. First end 276 maybe configured to attach to first rod 220 and second end 278 may beconfigured to attach to second rod 250. As seen in FIG. 3, dual springmember 270 facilitates attachment between first anchor 202 and secondanchor 204, generally creating some interdependence between anchors 202and 204.

In an alternative embodiment, other types of anchors may be used withthe dynamic stabilization device. In some cases, other types offasteners known in the art other than bone screws may be used to attachthe dynamic stabilization device to the adjacent vertebrae. In apreferred embodiment, one or more hooks may be used as anchors tominimize the trauma to the adjacent vertebrae during the fasteningprocess. In other embodiments, plates attached to the vertebral body maybe used as an alternative anchor.

FIGS. 4 and 5 are intended to illustrate an alternative embodiment ofdynamic stabilization device 100. Many of the components discussed withrespect to FIGS. 2 and 3 are identical for the current embodiment. Inthis embodiment, however, dynamic stabilization device 100 includesfirst hook 302 and second hook 304. In this preferred embodiment, firsthook 302 and second hook 304 may be configured to connect to thetransverse processes, lamina or spinous processes of adjacent vertebrae.

In some embodiments, a dynamic stabilization device may includeprovisions for locking a dual spring member into place. In some cases,this may be accomplished by including locking members. In a preferredembodiment, the locking members may be configured to limit the motion ofthe ends of the dual spring member.

Referring to FIG. 4, dynamic stabilization device 100 may include firstlocking member 402 and second locking member 404. In some embodiments,locking members 402 and 404 may be caps that are configured to slideover rods 220 and 250, respectively. In some cases, locking members 402and 404 may include provisions for locking or snapping into place overrods 220 and 250, respectively. In some embodiments, locking members 402and 404 may also be screwed onto rods 220 and 250 respectively.

Referring to FIG. 5, first locking member 402 may be disposed betweenfirst hook 302 and first end 276 of dual spring member 270. Thispreferred arrangement may prevent first end 276 of dual spring member270 from moving with respect to first rod 220. In particular, firstlocking member 402 may prevent outer spring 272 from moving at first end276. In some embodiments, second locking member 404 is disposed betweensecond hook 304 and second end 278 of dual spring member 270. Thisarrangement preferably prevents second end 278 from moving in a mannersimilar to the way that first locking member 402 prevents movement atfirst end 276.

It should be understood that locking members could be used in otherembodiments of a dynamic stabilization device. Although the currentembodiment includes locking members used with hooks, in otherembodiments, locking members could be used with any type of anchors fora dynamic stabilization device, including plates. Additionally, lockingmembers may be used with any type rods or dual spring member.

Preferably, a dual spring member may include provisions for increasedstructural stability. In some embodiments, the dual spring member maycomprise a single piece of material that is coiled into an outer springand an inner spring. In other embodiments, the dual spring member maycomprise two distinct springs.

FIG. 6 is an isometric view of a preferred embodiment of dual springmember 270. As previously mentioned, dual spring member 270 comprisesouter spring 272 and inner spring 274. In a preferred embodiment,springs 272 and 274 comprise a single material. In other words, firstend 276 of dual spring member 270 may comprise first discontinuous end280 and second discontinuous end 282, associated with outer spring 272and inner spring 274, respectively. Furthermore, second end 278 of dualspring member 270 does not include any discontinuous ends. Instead,outer spring 272 and inner spring 274 are joined directly at transitionregion 290.

It should be understood that inner spring 274 is shaded in FIG. 6 toemphasize inner spring 274 from outer spring 272. This shading is notintended to reflect any difference in material properties of springs 272and 274, or any other physical distinctions. In a preferred embodiment,outer spring 272 and inner spring 274 comprise a single, homogenousmaterial including substantially similar material properties. In analternative embodiment, however, it is possible that outer spring 272and inner spring 274 may be made of a distinct material.

It should be understood that while outer spring 272 and inner spring 274are joined in this embodiment, in other embodiments, springs 272 and 274may comprise separate springs that are not directly connected.Additionally, in some embodiments, springs 272 and 274 may be joined atboth ends of dual spring member 270. FIG. 7 is an alternative embodimentof dual spring member 270. In this embodiment, first discontinuous end280 and second discontinuous end 282 have been joined at first connectedregion 285. In some cases, ends 280 and 282 may be connected by anysuitable method. In some embodiments, a low temperature bondingtechnique may be used. In a preferred embodiment, first connected region285 forms a strong mechanical connection. With this alternativearrangement, stresses experienced by first end 276 and second end 278 ofdual spring member 270 may be substantially equal since both ends 276and 278 are closed with no discontinuous ends.

FIG. 8 is another embodiment of dual spring member 270. In thisembodiment, outer spring 272 and inner spring 274 are completelydisconnected. In some cases, outer spring 272 includes firstdiscontinuous end 280 and third discontinuous end 281. Also, innerspring 274 includes second discontinuous end 282 and fourthdiscontinuous end 283. With this arrangement, outer spring 272 and innerspring 274 may be free to move somewhat independently of one another.

Preferably, outer spring 272 and inner spring 274 share a common centralaxis 292. In other words, inner spring 274 is generally concentric withouter spring 272. In other embodiments, inner spring 274 could have acentral axis that is slightly misaligned with outer spring 272. Varyingthe position of inner spring 274 with respect to outer spring 272 mayfacilitate changing the flexibility properties of springs 272 and 274 inthe non-axial direction. In other words, modifying the orientation ofsprings 272 and 274 could allow for different bending properties of dualspring member 270. In some embodiments, springs 272 and 274 could becoiled in opposing directions. This alternative arrangement could helpprevent inter-digitation of springs 272 and 274.

Generally, an inner spring and an outer spring may have any crosssectional shape. In particular, the cross sectional shape of the springwire could be circular, triangular, rectangular as well as any otherpolygonal or irregular shape. Additionally, the cross sectional shape ofthe coil, which includes the windings of the spring wire, could have anyshape, including circular, triangular, rectangular as well as anypolygonal or irregular shape. It is also possible to provide inner andouter springs with different sized coils or wires. In other words, thewire diameters, shapes, configurations and sizes of the inner and outersprings may be different. By modifying the cross sectional shape and/ordiameters of the spring wires and the spring coils comprising the innerand outer springs, various mechanical properties of a dual spring membercan be modified.

In previous designs of spinal devices incorporating springs, theconnection point of the springs could be a weak point. Often, inprevious designs, springs may be attached to various rods or anchorsusing soldering techniques or other mechanical attachment techniques. Insome cases, springs may simply be disposed against stops that preventmotion of the ends.

Preferably, a dual spring member includes provisions for securing aconnection between the ends of the spring and the associated rods. Insome embodiments, these connections may be made by incorporatingthreaded portions onto the rods that are configured to receive the endsof the dual spring member. In a preferred embodiment, these threadedportions may naturally conform to the free or natural state of the coilsassociated with the dual spring member.

FIGS. 9 and 10 illustrate a preferred method of attaching dual springmember 270 to first rod 220 and second rod 250. Preferably, first rod220 includes first threaded portion 502. First threaded portion 502preferably has a larger diameter than extended portion 504 of first rod220. In a preferred embodiment, first threaded portion 502 may alsoinclude threading 506.

Preferably, first threaded portion 502 is configured to form a strongmechanical connection with first end 276 of dual spring member 270. Inthis embodiment, first threaded portion 502 may have a diameter D1 thatis approximately equivalent to the inner diameter D2 of inner spring 274at first end 276. Preferably, diameter D1 is associated with the basediameter of first threaded portion 502. In other words, diameter D1 isthe diameter of first threaded portion 502 when threading 506 isremoved. Additionally, threading 506 of first threaded portion 502 mayhave a diameter of D3 that is approximately equivalent to the innerdiameter D4 of outer spring 272 at first end 276. In some embodiments,the spacing L1 between threading 506 may also be approximately equal tothe spacing L2 between adjacent coils on inner spring 274.

Using this preferred arrangement, inner spring 274 may be configured towrap around first threaded portion 502, between threading 506, as seenin FIG. 10. This may be achieved by screwing first threaded portion 502together with inner spring 274. Furthermore, outer spring 272 may bewrapped around first threaded portion 502, tightly coiling around theouter surface of threading 506. In a preferred embodiment, outer spring272 may be fixed between coils 512 of inner spring 274, thus keepingouter spring 272 fixed in place at first end 276.

Preferably, second rod 250 includes second threaded portion 522configured to connect to second end 278 of dual spring member 270. Thismechanical connection is preferably formed in a substantially similarmanner to the connection between first threaded portion 502 and firstend 276 of dual spring member 270.

In an alternative embodiment, a dual spring member could be used thatincorporates two separate springs, as previously mentioned. Referring toFIGS. 11 and 12, an alternative embodiment of dual spring member 700preferably comprises inner spring 702 and outer spring 704 that arephysically separated. In order to incorporate two separate springs, athreaded portion of a rod should be modified to separately fasten toeach spring 702 and 704.

The method of connecting dual spring 700 proceeds in a similar manner tothe connection formed in the previous embodiment. In this case, however,first rod 220 preferably includes outer threaded portion 710 as well asinner threaded portion 712. In this embodiment, outer threaded portion710 is configured to receive outer spring 704 at first end 720 of dualspring member 700. Likewise, inner threaded portion 712 is preferablyconfigured to receive inner spring 702 at first end 720 of dual springmember 700.

Preferably, outer threaded portion 710 has a diameter D5 that is roughlyapproximate to the inner diameter D6 of outer spring 704 at first end720. Additionally, inner threaded portion 712 has a diameter D7 that isroughly approximate to the inner diameter D8 of inner spring 702 atfirst end 720. In both cases, the threaded portions are sized tosuitably engage and capture their respective springs. Preferably, secondrod 250 includes similar provisions for fastening to second end 722 ofdual spring member 700. With this arrangement, first end 720 and secondend 722 of dual spring member 700 may be securely fastened to rods 220and 250, respectively, as seen in FIG. 12.

In some embodiments, a dynamic stabilization device may include rodswith threading. In the embodiment shown in FIGS. 11 and 12, first rod220 includes first threading 750 and second rod 250 includes secondthreading 752. In this embodiment, first threading 750 and secondthreading 752 are oriented in opposing directions. Therefore, using thispreferred arrangement, first rod 220 and second rod 250 may be turnedwith respect to dual spring member 700 to modify the tension of thedynamic stabilization device. This configuration may be similar to aturn-buckle arrangement that is found in various mechanical systems.

FIG. 13 is a cross sectional view of another embodiment of a dual springmember connecting to rods of a dynamic stabilization device. In thisembodiment, dual spring member 450 comprises outer spring 452 and innerspring 454, which are completely disconnected from one another. In somecases, first rod 460 includes first outer threaded portion 462 forreceiving outer spring 452 at first side 480 of dual spring member 450.Likewise, first rod 460 includes first hollow portion 464 for receivinginner spring 454 at first side 480 of dual spring member 450. In asimilar manner, second rod 470 includes second outer threaded portion472 for receiving outer spring 452 at second side 482 of dual springmember 450. Also, second rod 470 includes second hollow portion 474 forreceiving inner spring 454 at second side 482 of dual spring member 450.This arrangement provides a method of connecting dual spring member 450to rods 460 and 470 when dual spring member 450 comprises twodisconnected springs.

In a similar manner to the previous embodiments, outer threaded portions462 and 472 may include grooves to receive outer spring 452. In somecases, hollow portions 464 and 474 may also include grooves to receiveinner spring 454. Using a grooved arrangement allows springs 452 and 454to be securely fastened with respect to rods 460 and 470. In otherembodiments, hollow portions 464 and 474 may not include grooves toallow the ends of inner spring 454 to move freely. In other embodiments,outer portions 462 and 472 may not include grooves. Generally, in theseembodiments, the outer spring 452 may be constrained with a cap. Inthese embodiments, the ends of inner spring 454 are preferablyconstrained within hollow portions 464 and 474.

Preferably, a dual spring member may include provisions for maintainingstrength and resiliency for extended periods of time. This is necessaryto ensure that stabilization to the spine is maintained over thelifetime of a patient who has a dynamic stabilization device implantedin their spine.

Preferably, the dual spring member includes provisions for preventingdifferential loading between the outer spring and the inner spring inorder to prevent premature failure due to overloading of one spring. Theterm “differential loading” as used through this detailed descriptionand in the claims refers to the tendency of one spring comprising a dualspring member to experience increased loads over a second spring. Inother words, in some dual spring systems, one spring may carry amajority of the total load applied to both springs. Instead, it ispreferable that a dual spring system include provisions for equallydividing the load between the two springs.

Referring back to FIGS. 9 and 10, in a preferred embodiment, outerspring 272 and inner spring 274 are configured to have equal workinglengths in order to reduce or eliminate differential loading. Workinglength is approximately equal to the length of the portion of the springthat is able to compress. Referring to FIG. 10, the working length forouter spring 272 and inner spring 274 are associated with the portionsof each spring that are free to bend and/or compress. In particular,first working portion 602 of outer spring 272 is shaded. Also, secondworking portion 604 of inner spring 274 is shaded.

The working length for each spring may be approximated by multiplyingthe diameter of each spring times the number of turns associated withfree portions of the spring. Since outer spring 272 has a diameter D4that is larger than the diameter D2 associated with inner spring 274, toachieve an approximately equal working length, inner spring 274 shouldinclude a larger number of turns along second working portion 604.

Using this preferred configuration, dual spring member 270 may beconfigured to reduce differential loading associated with outer spring272 and inner spring 274. This is an important feature for a dynamicstabilization device that may be used over an extended period of timesince it prevents one spring from wearing out too quickly due tooverloading. Furthermore, it should be understood that these samegeneral principles for reducing differential loads apply to alternativeembodiments that incorporate separate inner and outer springs.

Preferably, dynamic stabilization device 100 is preferably configured sothat dual spring member 270 never experiences a stress greater than apredetermined fatigue limit. The term ‘fatigue limit’ refers to amaximum amount of stress that may be applied to a material to ensurethat the material does not ever experience mechanical failure.

These various configurations of a dynamic fixation system may require adual spring member to undergo constant bending as well as other types ofstresses. FIGS. 14 and 15 depict embodiments of dual spring member 270experiencing bending. In FIG. 14, dual spring member 270 may compress atproximal side 1004 and expand under tension at distal side 1002, asspine 101 undergoes flexion. Additionally, in FIG. 15, dual springmember 270 may expand under tension at proximal side 1004 and compressat distal side 1002, as spine 101 undergoes extension. In thesepositions, dual spring member 270 may experience a maximum level of loadassociated with bending of dual spring member 270. Because dual springmember 270 is constantly undergoing various stresses, it is preferableto construct dynamic stabilization device 100 in a manner that preventsstressing dual spring member 270 to the point of failure. Generally,dynamic stabilization device 100 may be designed so that the lifeexpectancy of dual spring member 270 is effectively indefinite. In otherwords, dual spring member 270 is designed to last much longer than thelife expectancy of a human patient.

FIG. 16 illustrates an exemplary embodiment of a relationship betweenfatigue strength and number of stress cycles for dual spring member 270.Fatigue strength refers to the amount of stress applied to a material,and is characterized in the current embodiment in terms of Mega-Pascals(MPa). The number of stress cycles refers to the number of times amaterial can be stressed in a particular manner before undergoingfailure. In this embodiment, the stress cycle represents a compressionand re-expansion of proximal side 1004 of dual spring member 270 duringflexion of spine 101, as depicted in FIG. 14. Referring to curve 900, at600 MPa, spring 270 may undergo 10,000 cycles (compressions) beforefailing. Furthermore, at 400 MPa, spring 270 may undergo approximately1,000,000 cycles (compressions) before failing.

Curve 900 follows a general pattern for a class of materials containingiron, such as steel. In particular, this relationship includes fatiguelimit 902 at approximately 300 MPa. Fatigue limit 902 represents astress level below which dual spring member 270 may be indefinitelyfatigued without failing. Therefore, as long as dual spring member 270is designed so that during operation it does not undergo compressionstresses at or above 300 MPa, dual spring member 270 may continuefunctioning indefinitely without undergoing failure.

Preferably, to allow for some variation in experimentally determinedfatigue curves as well as to provide for some change in operatingconditions, a dynamic stabilization device may be defined with aneffective fatigue limit. An effective fatigue limit may be used toensure safe use of a mechanical device by designing the device tooperate at stress levels far below the fatigue limit. This built insafety factor allows for some error in designing the device withoutrisking mechanical failure. In this preferred embodiment, effectivefatigue limit 904 has a value of 150 MPa, which is less than 50% offatigue limit 902. Therefore, using this preferred arrangement, dynamicstabilization device 100 may be designed so that dual spring member 270never experiences stresses above effective fatigue limit 904 to ensureindefinite durability of dual spring member 270.

It should be understood that curve 900 is only intended to be exemplary.The values discussed here, especially the value 150 MPa for an effectivefatigue limit associated with compressive and tension forces, are onlyintended to illustrate the concept of choosing an effective fatiguelimit that is far less (around 50%) than the fatigue limit. Furthermore,although this embodiment illustrates an effective fatigue limit for thecompression and tension of dual spring member 270, effective fatiguelimits for tensile stresses and shear stresses are also preferablyconsidered in constructing dynamic stabilization device 100. In otherwords, the stresses applied to dual spring member 270 as it is stretchedand bent should also be below associated effective fatigue limits at alltimes to ensure that dual spring member 270 never undergoes failure.

Using this preferred arrangement, dynamic stabilization device 100 maybe configured to function for an indefinite period of time by ensuringthat dual spring member 270 never experiences stresses greater thaneffective fatigue limit 904. In other words, dynamic stabilizationdevice 100 may be designed so that under normally expected physiologicalloads, dual spring member 270 will not be stressed beyond effectivefatigue limit 904. This feature of durability when coupled with theprovision to reduce differential loading preferably helps to maintainthe structural integrity of dynamic stabilization device 100indefinitely. This preferably allows dynamic stabilization device 100 tofunction for the lifetime of a patient, ensuring the stability of thespine over this period.

Preferably, a dynamic stabilization device includes provisions forsecurely fastening rods in place with respect to bone anchors as well asprovisions for facilitating fine tuned adjustments to a spanning lengthbetween two anchors. FIG. 17 illustrates a preferred embodiment of theconnection between first rod 220 and first anchor 202. First rod 220 isconfigured to insert into first slotted portion 216 of first anchor head214. In a preferred embodiment, first rod 220 includes ridges 1202 thatare configured to rest within first recesses 1204 disposed within firstslotted portion 216.

In this preferred embodiment, first recesses 1204 of first slottedportion 216 may be disposed within first drive receiving surface 210. Inparticular, first slotted portion 216 cuts through second flat side 1250of first drive receiving surface 210. Similarly, first slotted portion216 cuts through a flat side disposed opposite of second flat side 1250.This preferred arrangement allows for increased strength and durabilityof dynamic stabilization device 100, as first rod 220 may be fixed inplace just above where first anchor 202 is driven into bone, thusreducing the moment arm and the forces experienced by dynamicstabilization device 100. In still other embodiments, first recessed1204 and first slotted portion 216 may be disposed above first drivereceiving surface 210. This arrangement may provide an increased momentarm, which can be used to correct deformities.

Once first rod 220 is inserted into first slotted portion 216, first cap222 may be attached to first anchor head 214. In this embodiment, firstinner cap 224 is disposed within first outer cap 226. First inner cap224 includes second recesses 1206 configured to engage ridges 1202.Additionally, first outer cap 226 includes cap threading 1208 configuredto screw onto first anchor threading 218. Therefore, as first cap 222 isscrewed into place, first rod 220 is locked into place with respect tofirst anchor 202.

Although this embodiment includes the attachment of first rod 220 tofirst anchor 202, it should be understood that a similar procedure isused to attach second rod 250 to second anchor 204. Using the preferredconfiguration, first anchor 202 and second anchor 204 can be secured viarods 220 and 250, respectively, to dual spring member 270.

Preferably, this method of attachment allows a surgeon to fine tune thedistance between first anchor 202 and second anchor 204, as the anchors202 and 204 may be moved with respect to rods 220 and 250 in incrementsassociated with the distance between ridges 1202. In some embodiments,the spacing between ridges 1202 may be made small so that very fineadjustments may be made. Furthermore, because of the nature of theconnection, rods 220 and 250 will not slip with respect to anchors 202and 204, thus preserving the initially selected length indefinitely.

In some embodiments, the shape of ridges associated with a rod may bemodified. In some cases, the cross-sectional shape of the ridges couldbe sinusoidal, box-like, triangular, rounded or any pattern. In othercases, a rod may not include any ridges, but instead may be smooth. Instill other embodiments, a rod may include various textures such as aknurling pattern. In these embodiments, components of the dynamicstabilization device that are configured to engage the rod may alsoinclude textured patterns to provide frictional connections between theconfronting surfaces of the rod and the adjacent components.Additionally, a rod could include threading so that the distance betweenthe rods can be moved by turning them, as was discussed in a previousembodiment.

FIGS. 18 and 19 illustrate alternative embodiments of a rod used with adynamic stabilization device. In FIG. 18, first rod 220 preferablyincludes smooth surface 1802. In this embodiment, first cap 222 may beconfigured with smooth recess 1806 to receive smooth surface 1802.Likewise, first slotted portion 216 may include smooth recessed portion1804. In FIG. 19, first rod 220 preferably includes threading 1902.Preferably, first cap 222 is configured with first threaded recesses1906 to receive threading 1902. Likewise, first slotted portion 216 mayinclude second threaded recesses 1904 to receive threading 1902. Withthis arrangement, first rod 220 may be configured to turn with respectto first cap 222 and first anchor 202. This threaded arrangement allowsfor additional adjustments of first rod 220 after the dynamicstabilization device has been assembled. Although only first rod 220 isshown in FIGS. 18 and 19, these alternative surfaces could also beapplied to second rod 250.

Generally, dual spring member 270 will undergo various stressesfollowing implantation into a spine. Initially, referring to FIG. 1,dynamic stabilization device 100 is in a rest position associated with agenerally straightened position of spine 101. In this rest position,dual spring member 270 may experience some minimum level of load due tothe stresses applied to dual spring member 270 in this position.However, in other embodiments, it is possible to install stabilizationdevice 100 with either an initial compression or tension. In otherwords, stabilization device 100 includes some kind of pre-stress, eitherresidual compression or residual tension. It is also possible to loaddifferent portions of the vertebral bodies with unequal or oppositeforces. This arrangement may be used to correct or treat deformities,such as scoliosis or spondylolisthesis.

For example, to correct scoliosis, a stabilization system including twostabilization devices may be attached to two adjacent vertebrae. Thesetwo dynamic stabilization devices may include different residualstresses, one stabilization device including residual tension and thesecond stabilization device including residual compression. Thisarrangement can apply a rotational force to the adjacent vertebraewhereby scoliosis may be corrected or stabilized.

In another example, to correct spondylolisthesis, a stabilization systemincluding two stabilization devices may be attached to two adjacentvertebrae. These two dynamic stabilization devices may includesubstantially similar residual stresses, both stabilization devicesincluding residual tension. This arrangement can apply a translationalforce to the adjacent vertebrae whereby spondylolisthesis may becorrected or stabilized. This motion can be demonstrated by comparingFIG. 14 to FIG. 1, where FIG. 14 is the initial position, and FIG. 1 isthe resulting, treated position.

In another example, to correct spinal stenosis, a stabilization systemincluding two stabilization devices may be attached to two adjacentvertebrae. These two dynamic stabilization devices may includesubstantially similar residual stresses, both stabilization devicesincluding residual compression. This arrangement can apply a flexionforce to the adjacent vertebrae whereby stenosis may be corrected orstabilized. This motion can be demonstrated by comparing FIG. 15 to FIG.1, where FIG. 15 is the initial position, and FIG. 1 is the resulting,treated position. In another case, this motion can be demonstrated bycomparing FIG. 1 to FIG. 14, where FIG. 1 is the initial position, andFIG. 14 is the resulting, treated position.

In a final example, to correct scoliosis, a stabilization systemincluding two stabilization devices may be attached to two adjacentvertebrae. These two dynamic stabilization devices may include twodifferent residual stresses, with a first stabilization device includinga residual compression and a second stabilization device including aresidual tension. This arrangement can apply both compression andtension forces to the adjacent vertebrae whereby scoliosis may becorrected or stabilized.

Referring to FIG. 20, second vertebrae 2004 may be bent or curved withrespect to first vertebrae 2002 along a portion of spine 2000 that isdeformed due to scoliosis. Although this exemplary embodiment includes arightwards leaning of second vertebrae 2004, it should be understoodthat in other cases, a vertebrae could be leftwards leaning.Additionally, in some cases, the vertebrae may also lean forwards ortwist with respect to an adjacent vertebrae. Each of these differentconfigurations associated with scoliosis may be generally treated usingsimilar principles as those discussed for the current embodiment.

In some embodiments, first stabilization device 2010 may be associatedwith first side 2006 of spine 2000. In this embodiment, firststabilization device 2010 has a first initial position 2020 that isassociated with a free or unstressed state of first dual spring member2011. Additionally, second stabilization device 2012 may be associatedwith second side 2008 of spine 2000. In this embodiment, secondstabilization device 2012 has a second initial position 2022 that isassociated with a free or unstressed state of second dual spring member2013.

In some embodiments, stabilization devices 2010 and 2012 may bepre-stressed before being attached to vertebrae 2002 and 2004. In thecurrent embodiment, first stabilization device 2010 may undergo apre-tension that causes first stabilization device 2010 to stretch fromfirst initial position 2020 to first stressed position 2030. At thispoint, first dual spring member 2011 has preferably expanded from firstoriginal length L1 to first modified length L2. Following this, firststabilization device 2010 may be attached to first vertebrae 2002 atfirst attachment point 2042 and to second vertebrae 2004 at secondattachment point 2041.

In this embodiment, second stabilization device 2012 may undergo apre-compression that squeezes second stabilization device 2012 from asecond initial position 2022 to a second stressed position 2032. At thispoint, second dual spring member 2013 has preferably contracted fromsecond original length L3 to second modified length L4. Following this,second stabilization device 2012 may be attached to first vertebrae 2002at third attachment point 2044 and to second vertebrae 2004 at fourthattachment point 2043.

Referring to FIG. 21, as stabilization devices 2010 and 2012 areattached to vertebrae 2002 and 2004 in pre-stressed conditions, dualspring members 2011 and 2013 may be configured to apply compressiveforces and tension forces, respectively, to vertebrae 2002 and 2004. Inparticular, first dual spring member 2011 may act to pull firstvertebrae 2002 and 2004 together at first side 2006. Additionally,second dual spring member 2013 may act to separate vertebrae 2002 and2004 at second side 2008. This general arrangement may help to realignspine 2000 over time, as vertebrae 2002 and 2004 are generallystraightened with respect to one another.

FIG. 22 is a preferred embodiment of vertebrae 2002 and 2004 in properalignment. In some cases, when vertebrae 2002 and 2004 have beenproperly aligned, stabilization devices 2010 and 2012 return to initialpositions 2020 and 2022, respectively. In these initial positions 2020and 2022, dual spring members 2011 and 2013 may have first initiallength L1 and second initial length L3, respectively. This arrangementmay prevent overcorrection since dual spring members 2011 and 2013 areno longer stressed and therefore will no longer apply forces tovertebrae 2002 and 2004 once vertebrae 2002 and 2004 have been properlyaligned. However, in some cases, for example, in a fixed deformity, evenafter dual spring members 2011 and 2013 return to their initial lengths,the dual spring members may continue to apply a corrective force tovertebrae 2002 and 2004.

Using these various provisions discussed throughout this detaileddescription preferably improves the performance of a dynamicstabilization device. In particular, the resistance to differentialloading and the design to maintain stresses below a predefined effectivefatigue limit may increase the lifetime of dynamic stabilization device100.

The materials used to make a dynamic stabilization device may vary fromone embodiment to another embodiment. Preferably, materials used toconstruct the various components are rigid and may be designed to endurethe harsh environment of the human body. Materials used for anchors(including screws and hooks), rods and various other components are wellknown in the art. Preferably, the materials used to construct a dualspring member are also relatively flexible to provide for somedeflection of the inner and outer springs.

While various embodiments of the invention have been described, thedescription is intended to be exemplary, rather than limiting and itwill be apparent to those of ordinary skill in the art that many moreembodiments and implementations are possible that are within the scopeof the invention. Accordingly, the invention is not to be restrictedexcept in light of the attached claims and their equivalents. Also,various modifications and changes may be made within the scope of theattached claims.

What is claimed is:
 1. A method for treating a deformity of a spine, thespine comprising a plurality of vertebrae each having a first lateralportion, a central spinous process portion, and a second lateral portionon a side of the central spinous process portion opposite to the firstlateral portion, the method comprising: attaching a first anchor to thefirst lateral portion of a first vertebra of the spine; attaching asecond anchor to the first lateral portion of a second vertebra of thespine; attaching a third anchor to the second lateral portion of thefirst vertebra; attaching a fourth anchor to the second lateral portionof the second vertebra; pre-stressing a first dual spring member betweenthe first anchor and the second anchor and attaching the first dualspring member to the first anchor and the second anchor so as to apply afirst force between the first lateral portion of the first vertebra andthe first lateral portion of the second vertebra, wherein the first dualspring member comprises a first outer coil spring and a first inner coilspring inside the first outer coil spring, wherein the first outer coilspring has a first working length, wherein the first working length is alength through which the first outer coil spring extends through turnsalong a free portion of the first outer coil spring over which the firstouter coil spring is free to bend or compress, wherein the first innercoil spring has a second working length, wherein the second workinglength is a length through which the first inner coil spring extendsthrough turns along a free portion of the first inner coil spring overwhich the first inner coil spring is free to bend or compress, andwherein the first working length is substantially equal to the secondworking length; pre-stressing a second dual spring member between thethird anchor and the fourth anchor and attaching the second dual springmember to the third anchor and the fourth anchor so as to apply a secondforce between the second lateral portion of the first vertebra and thesecond lateral portion of the second vertebra, wherein the second dualspring member comprises a second outer coil spring and a second innercoil spring inside the second outer coil spring, wherein the secondouter coil spring has a third working length, wherein the third workinglength is a length through which the second outer coil spring extendsthrough turns along a free portion of the second outer coil spring overwhich the second outer coil spring is free to bend or compress, whereinthe second inner coil spring has a fourth working length, wherein thefourth working length is a length through which the second inner coilspring extends through turns along a free portion of the second innercoil spring over which the second inner coil spring is free to bend orcompress, and wherein the third working length is substantially equal tothe fourth working length; and treating the deformity by application ofthe first force and the second force.
 2. The method of claim 1, whereinthe deformity comprises scoliosis, wherein pre-stressing the first dualspring member comprises stretching the first dual spring member betweenthe first anchor and the second anchor and attaching the first dualspring member to the first anchor and the second anchor so as to pullthe first lateral portion of the first vertebra and the first lateralportion of the second vertebra toward each other, wherein pre-stressingthe second dual spring member comprises compressing the second dualspring member between the third anchor and the fourth anchor andattaching the second dual spring member to the third anchor and thefourth anchor so as to push the second lateral portion of the firstvertebra and the second lateral portion of the second vertebra away fromeach other, and wherein the pre-stressing of the first dual springmember and the second dual spring member applies a force between thefirst vertebra and the second vertebra to treat the scoliosis.
 3. Themethod of claim 1, wherein the deformity comprises spondylolisthesis,wherein pre-stressing the first dual spring member comprises stretchingthe first dual spring member between the first anchor and the secondanchor and attaching the first dual spring member to the first anchorand the second anchor so as to pull the first lateral portion of thefirst vertebra and the first lateral portion of the second vertebratoward each other, wherein pre-stressing the second dual spring membercomprises stretching the second dual spring member between the thirdanchor and the fourth anchor and attaching the second dual spring memberto the third anchor and the fourth anchor so as to pull the secondlateral portion of the first vertebra and the second lateral portion ofthe second vertebra toward each other, and wherein the pre-stressing ofthe first dual spring member and the second dual spring member applies aforce between the first vertebra and the second vertebra to treat thespondylolisthesis.
 4. The method of claim 1, wherein the deformitycomprises stenosis, wherein pre-stressing the first dual spring membercomprises compressing the first dual spring member between the firstanchor and the second anchor and attaching the first dual spring memberto the first anchor and the second anchor so as to push the firstlateral portion of the first vertebra and the first lateral portion ofthe second vertebra away from each other, wherein pre-stressing thesecond dual spring member comprises compressing the second dual springmember between the third anchor and the fourth anchor and attaching thesecond dual spring member to the third anchor and the fourth anchor soas to push the second lateral portion of the first vertebra and thesecond lateral portion of the second vertebra away from each other, andwherein the pre-stressing of the first dual spring member and the seconddual spring member applies a force between the first vertebra and thesecond vertebra to treat the stenosis.
 5. The method of claim 1, furthercomprising reducing differential loading between the first outer coilspring and the first inner coil spring by configuring the first andsecond working lengths to be approximately equal.
 6. The method of claim5, further comprising reducing differential loading between the secondouter coil spring and the second inner coil spring of the second dualspring member by configuring the third and fourth working lengths to beapproximately equal.
 7. The method of claim 1, further comprisingconfiguring the first outer coil spring to have a first wire diameteralong the first working length and the first inner coil spring to have asecond wire diameter along the second working length, wherein the firstwire diameter is different from the second diameter.
 8. A method fortreating a deformity of a spine, the spine comprising a plurality ofvertebrae each having a first lateral portion, a central spinous processportion, and a second lateral portion on a side of the central spinousprocess portion opposite to the first lateral portion, the methodcomprising: attaching a first anchor to the first lateral portion of afirst vertebra of the spine; attaching a second anchor to the firstlateral portion of a second vertebra of the spine; attaching a thirdanchor to the second lateral portion of the first vertebra; attaching afourth anchor to the second lateral portion of the second vertebra;pre-stressing a first dual spring member between the first anchor andthe second anchor and attaching the first dual spring member to thefirst anchor and the second anchor so as to apply a first force betweenthe first lateral portion of the first vertebra and the first lateralportion of the second vertebra; pre-stressing a second dual springmember between the third anchor and the fourth anchor and attaching thesecond dual spring member to the third anchor and the fourth anchor soas to apply a second force between the second lateral portion of thefirst vertebra and the second lateral portion of the second vertebra;and treating the deformity by application of the first force and thesecond force, wherein the first dual spring member comprises a first rodat a first end and a second rod at a second end opposite to the firstend, wherein the first rod attaches to the first anchor and the secondrod attaches to the second anchor, wherein the first rod and the secondrod each have a plurality of first incremental adjustment structures,wherein the first anchor and the second anchor each have a plurality ofsecond incremental adjustment structures complementary to the firstincremental adjustment structures, wherein pre-stressing the first dualspring member between the first anchor and the second anchor andattaching the first dual spring member to the first anchor and thesecond anchor comprises moving the first anchor and the second anchorrelative to the respective first and second rods in incrementsassociated with the plurality of first incremental adjustment structuresand the plurality of second incremental adjustment structures.
 9. Themethod of claim 8, wherein the first incremental adjustment structurescomprise separate annular ridges around the first rod and the secondrod, and wherein the second incremental adjustment structures compriseinternal annular recesses on the first anchor and the second anchor. 10.The method of claim 9, wherein the first anchor and the second anchoreach comprise an anchor head and a cap removably attachable to theanchor head, wherein, when attached, the cap and the anchor head definethe internal annular recesses, wherein the method further comprises:locking in place the first dual spring member to the first anchor byfixing the cap of the first anchor to the anchor head of the firstanchor, with the separate annular ridges of the first rod engaged withthe internal annular recesses of the first anchor, and locking in placethe first dual spring member to the second anchor by fixing the cap ofthe second anchor to the anchor head of the second anchor, with theseparate annular ridges of the second rod engaged with the internalannular recesses of the second anchor.
 11. The method of claim 9,wherein the method further comprises adjusting a distance between thefirst anchor and the second anchor between which the dual spring memberis connected to the first anchor and the second anchor, by moving thefirst rod relative to the first anchor in increments of distancesbetween the separate annular ridges of the first rod and by moving thesecond rod relative to the second anchor in increments of distancesbetween the separate annular ridges of the second rod.
 12. A method fortreating a deformity of a spine, the spine comprising a plurality ofvertebrae each having a first lateral portion, a central spinous processportion, and a second lateral portion on a side of the central spinousprocess portion opposite to the first lateral portion, the methodcomprising: attaching a first anchor to the first lateral portion of afirst vertebra of the spine; attaching a second anchor to the firstlateral portion of a second vertebra of the spine; attaching a thirdanchor to the second lateral portion of the first vertebra; attaching afourth anchor to the second lateral portion of the second vertebra;pre-stressing a first dual spring member between the first anchor andthe second anchor and attaching the first dual spring member to thefirst anchor and the second anchor so as to apply a first force betweenthe first lateral portion of the first vertebra and the first lateralportion of the second vertebra; pre-stressing a second dual springmember between the third anchor and the fourth anchor and attaching thesecond dual spring member to the third anchor and the fourth anchor soas to apply a second force between the second lateral portion of thefirst vertebra and the second lateral portion of the second vertebra;and treating the deformity by application of the first force and thesecond force, wherein the first dual spring member comprises a first rodat a first end and a second rod at a second end opposite to the firstend, wherein the first rod attaches to the first anchor and the secondrod attaches to the second anchor, wherein the first rod and the secondrod each have threading, wherein the first anchor and the second anchoreach have threading recesses configured to receive the respectivethreading of the first rod and the second rod, wherein pre-stressing thefirst dual spring member between the first anchor and the second anchorand attaching the first dual spring member to the first anchor and thesecond anchor comprises rotationally displacing the threading of thefirst rod and the threading recesses of the first anchor relative toeach other, and rotationally displacing the threading of the second rodand the threading recesses of the second anchor relative to each other.13. The method of claim 12, wherein the first anchor and the secondanchor each comprise an anchor head and a cap removably attachable tothe anchor head, wherein, when attached, the cap and the anchor headdefine a slot having the threading recesses, wherein the method furthercomprises: attaching the cap of the first anchor to the anchor head ofthe first anchor to define the slot of the first anchor, securing thefirst rod within the slot of the first anchor, with the threading of thefirst rod engaging the threading recesses of the slot of the firstanchor, attaching the cap of the second anchor to the anchor head of thesecond anchor to define the slot of the second anchor, and securing thesecond rod within the slot of the second anchor with the threading ofthe second rod engaging the threading recesses of the slot of the secondanchor.
 14. The method of claim 13, wherein the first rod defines afirst longitudinal axis, wherein the cap of the first anchor includesfirst threading and the anchor head of the first anchor includes secondthreading, wherein the first threading engages the second threadingaround a second longitudinal axis, wherein the first longitudinal axisis approximately perpendicular to the second longitudinal axis, andwherein attaching the cap of the first anchor to the anchor head of thefirst anchor comprises screwing the first threading of the cap of thefirst anchor over the second threading of the anchor head of the firstanchor.
 15. The method of claim 12, wherein the method further comprisesadjusting a distance between the first anchor and the second anchorbetween which the dual spring member is connected to the first anchorand the second anchor, by turning the first rod relative to thethreading recesses of the first anchor and by turning the second rodrelative to the threading recesses of the second anchor.
 16. The methodof claim 12, wherein the threading of the first rod and the threading ofthe second rod are oriented in opposite directions, and wherein themethod further comprises adjusting a distance between the first anchorand the second anchor between which the dual spring member is connectedto the first anchor and the second anchor by turning the first dualspring member and its attached first and second rods in unison.
 17. Amethod for treating a deformity of a spine, the spine comprising aplurality of vertebrae each having a first lateral portion, a centralspinous process portion, and a second lateral portion on a side of thecentral spinous process portion opposite to the first lateral portion,the method comprising: attaching a first anchor to the first lateralportion of a first vertebra of the spine; attaching a second anchor tothe first lateral portion of a second vertebra of the spine; attaching athird anchor to the second lateral portion of the first vertebra;attaching a fourth anchor to the second lateral portion of the secondvertebra; pre-stressing a first dual spring member between the firstanchor and the second anchor and attaching the first dual spring memberto the first anchor and the second anchor so as to apply a first forcebetween the first lateral portion of the first vertebra and the firstlateral portion of the second vertebra; pre-stressing a second dualspring member between the third anchor and the fourth anchor andattaching the second dual spring member to the third anchor and thefourth anchor so as to apply a second force between the second lateralportion of the first vertebra and the second lateral portion of thesecond vertebra; and treating the deformity by application of the firstforce and the second force, wherein the first dual spring membercomprises an inner spring, an outer spring around the inner spring, afirst rod having first grooves, and a second rod having second grooves,wherein the method further comprises threading coils of a first end ofthe first dual spring member onto the first grooves of the first rod,and threading coils of a second end of the first dual spring member ontothe second grooves of the second rod.
 18. The method of claim 17,further comprising: attaching a first locking member to the first rodadjacent to the first grooves of the first rod, wherein the firstlocking member contacts the threading coils of the first end of thefirst dual spring member to limit further motion of the first end of thefirst dual spring member onto the first rod, and attaching a secondlocking member to the second rod adjacent to the second grooves of thesecond rod, wherein the second locking member contacts the threadingcoils of the second end of the first dual spring member to limit furthermotion of the second end of the first dual spring member onto the secondrod.
 19. The method of claim 17, wherein the first grooves of the firstrod comprise inner grooves and outer grooves, and wherein threading thecoils of the first end of the first dual spring member onto the firstgrooves of the first rod comprises threading coils of the inner springonto the inner grooves and threading coils of the outer spring onto theouter grooves.
 20. A method for treating a deformity of a spine, thespine comprising a plurality of vertebrae, the method comprising:attaching a first anchor to a first vertebra of the spine; attaching asecond anchor to a second vertebra of the spine; pre-stressing a dualspring member between the first anchor and the second anchor andattaching the dual spring member to the first anchor and the secondanchor so as to apply a force between the first vertebra and the secondvertebra that treats the deformity, wherein the first dual spring membercomprises an outer coil spring and an inner coil spring inside the outercoil spring; and reducing differential loading between the outer springand the inner spring by configuring working lengths of the outer springand the inner spring to be approximately equal, wherein a working lengthof a spring comprises a length of spring coil along a portion of thespring over which the spring coil is able to bend or compress.
 21. Amethod for treating a deformity of a spine, the spine comprising aplurality of vertebrae, the method comprising: attaching a first anchorto a first vertebra of the spine; attaching a second anchor to a secondvertebra of the spine; placing a dual spring member between the firstanchor and the second anchor, wherein: the dual spring member comprisesa first rod at a first end and a second rod at a second end opposite tothe first end, the first rod is disposed at the first anchor and thesecond rod is disposed at the second anchor, the first rod and thesecond rod each have a plurality of first incremental adjustmentstructures, and the first anchor and the second anchor each have aplurality of second incremental adjustment structures complementary tothe first incremental adjustment structures, pre-stressing the dualspring member between the first anchor and the second anchor by movingthe first anchor and the second anchor relative to the respective firstand second rods in increments associated with the plurality of firstincremental adjustment structures and the plurality of secondincremental adjustment structures; and locking in place the dual springmember to the first anchor and the second anchor so as to apply a forcebetween the first vertebra and the second vertebra that treats thedeformity.
 22. The method of claim 21, wherein the first incrementaladjustment structures comprise separate annular ridges around the firstrod and the second rod, and wherein the second incremental adjustmentstructures comprise internal annular recesses on the first anchor andthe second anchor, and wherein moving the first anchor and the secondanchor relative to the respective first and second rods in incrementsassociated with the plurality of first incremental adjustment structuresand the plurality of second incremental adjustment structures comprises:moving the first rod relative to the first anchor in increments ofdistances between the separate annular ridges of the first rod, andmoving the second rod relative to the second anchor in increments ofdistances between the separate annular ridges of the second rod.